A printed circuit board (PCB) may include one or more material layers that mechanically support and electrically connect electronic components using conductive pathways. The conductive pathways may be etched from copper sheets laminated onto a non-conductive substrate. The pathways may be organized as a number of layers on the PCB in order to increase the signal transmission density of the PCB. A PCB, after populating with electronic components, is commonly referred to as a printed circuit assembly (PCA).
PCBs may be used in high frequency applications. For example, a PCB may be populated with an integrated circuit used to enable high speed serial links to and from the PCB. In these situations, the PCB can experience significant signal integrity disturbances as a result of the unused portions of through-holes and vias, called stubs, that extend past the last connected layer of the PCB. The stubs can present resonances, impedance discontinuities, and increase the loss of the channel, thereby limiting performance.
For low density and low data rate serial links (e.g., <5 Gb/s), the signal lines are commonly routed on thin PCB substrates with Ball Grid Arrays (BGAs) connected to short vias, and can tolerate small stubs and present few challenges with power delivery. As the serial link speed and density increase, the PCB substrate thickness may be increased to accommodate more signal lines, which can in turn cause an increase in the stub length and a corresponding increase in signal integrity disturbances.
One technique for reducing stub length is known as backdrilling. Backdrilling is a controlled depth drilling (CDD) technique that removes stubs with numerically controlled drill equipment. In high density PCBs, backdrilling can be problematic because the clearance required for the drill bit may dictate that relatively large areas of metal conductive material, that could otherwise be used for signal lines or power distribution, must be removed. Backdrilling may thus limit the density of the PCB.